1. Field of the invention
The present invention concerns an inductive proximity sensor of the type including a resonant circuit including a sensing coil, an oscillator driving the resonant circuit at a predetermined frequency and a circuit for processing the signal generated by the oscillatory circuit in response to a metal object.
2. Description of the Prior Art
Ferrous metal (e.g. iron or steel) objects usually have a high magnetic permeability and a moderate resistivity. Non-ferrous metal (e.g. aluminum, copper, brass, etc) objects have a very low permeability (.mu..sub.r .congruent.1) and a very low resistivity. Metal objects in the field of the sensing coil of a normal proximity sensor therefore cause lower losses if they are made of non-ferrous metal; accordingly, the range at which these sensors can sense non-ferrous objects is much less than that for ferrous objects.
Specific provisions are therefore required if it is to be possible to sense ferrous and non-ferrous objects at the same distance. Patent DE-32 28 524 describes a sensor of the type indicated above and able to detect ferrous and non-ferrous objects at the same range. However, a sensor of this kind has the drawback of not enabling selective sensing of ferrous metal objects and non-ferrous metal objects.
An object of the invention is to provide an inductive proximity sensor which can sense at the same range and distinguish between ferrous metal objects and non-metal ferrous objects, using simple means and with highly reliable differentiation.